1. Field of the Invention
The present invention relates to correction of rotational unbalance of an impeller in a fan and an unbalance correction method for the fan.
2. Description of the Related Art
Electronic devices are provided with fans for dissipating heat generated in the electronic devices. Recently, the amount of the generated heat continues to increase with the improvement of the performance of the electronic devices, thus requiring the fans to have a higher performance. In order to achieve this, both flow-rate characteristics and static-pressure characteristics have to be improved. This can be achieved by driving fans to rotate at high speeds. On the other hand, reduction of both noise and vibration are required for many electronic devices used in homes and offices.
A rotating part of a fan, i.e., a rotor, includes a driving magnet, a rotor yoke, a shaft, and an impeller having a plurality of blades. The rotor rotates around the shaft as a rotation axis. When rotation of the rotor is unbalanced, the rotor vibrates during rotation. The magnitude of the vibration of the rotor is small in a case where the degree of unbalance is small or the rotor rotates at low speeds. However, in a case where the degree of unbalance is large or the rotor rotates at high speeds, the magnitude of the vibration of the rotor is large and the vibration is transferred to an electronic device in which the fan is incorporated. The transferred vibration adversely affects the electronic device, and causes resonance with other components in the electronic device. The resonance may generate a harsh noise. In order to avoid this problem, it is necessary to reduce the degree of unbalance of the rotor to a minimum.
In accordance with a conventional technique, the rotational unbalance of the rotor of the fan is corrected in the following manner. (1) A value indicating the degree of rotational unbalance of the assembled rotor is measured. Hereinafter, this value is referred to as “unbalance value”. (2) Balance weight in the form of paste is applied to a portion of the rotor to correct the unbalance. (3) The unbalance value is measured again, and steps (1) and (2) are repeated until the measured unbalance value is reduced below a predetermined acceptable level. (4) When the measured unbalance value is reduced below the predetermined acceptable level, the rotor is baked or left in an atmosphere in order to remove a vaporizing component in the balance weight.
However, this unbalance correction technique requires a considerable number of man-hours. Moreover, to leave the rotor in an atmosphere or bake the rotor to cause vaporization of the vaporizing component takes a considerable amount of time. Furthermore, the precision of the correction depends on the ability of the operator who performs the correction.